Since the passage of the Clean Water Act in 1972 the federal government through the Environmental Protection Agency (EPA) has mandated progressively tighter controls over the quantities of pollutants and contaminants that are allowed to be released into our nation's waters. These progressively tighter mandates also encompass control of peak flows and/or volumes and the rate at which they can be discharged into existing water ways or drainage infrastructures. These resulting mandates and controls have given birth to new programs and procedures collectively referred to as storm water management.
Devices, systems and procedures that remove or reduce the pollutants and contaminates and/or control peak flows and volumes are often referred to as best management practices (BMPs). The BMPs can utilize natural, artificial or man-made techniques, and even combinations of either and/or both. Some examples of these BMPs include trash filters, sedimentation basins, retention and detention ponds, wetlands, infiltration trenches, grass swales, various types of media filters, and various types of natural filter systems including sand filters, and aggregate filters including natural and artificial wetlands.
BMPs typically use one or more mechanisms to remove the pollutants and contaminates. These mechanisms include sedimentation, filtration, absorption, adsorption, flocculation, stripping, leaching, bioremediation, and chemical processes including oxidation reduction, ion exchange, and precipitation.
Furthermore, storm water treatment systems can also be classified in relationship to the treatment level in which they are being used. In this respect, the term treatment is generally used to describe the unit processes that are used to reduce the quantities of pollutants and containments in storm water runoff.
For example, basic or pre-treatment typically refers to the removal of gross solids, sediments and larger debris through the processes of settling and screening. Enhanced or advanced treatment typically refers to processes or reducing targeted pollutants.
Filtration is a common form of enhanced treatment for storm water. Filtration utilizes a combination of physical, chemical, and biological processes. Types of filtration greatly vary dependent on the media use. Medias can be both inert and/or sorbent and are also strongly linked to natural biological processes that thrive in and/or around the media environment. Advanced filtration techniques especially include chemical and biological processes and generally include, but are not limited to processes that bring storm water in contact with plants including both macrophytes and microphytes. And the plants are both visible and invisible to the naked eye.
The reduction of nutrients that are conveyed via storm water runoff are in the forefront of the various pollutants of concern. For example; the EPA has mandated that the state of Florida reduce the overall pollutant discharge of the state by approximately ⅓. To this end the EPA has established a numeric nutrient criteria for all water bodies of the state for total phosphorus (TP) and total nitrogen (TN). These nutrient numeric values cannot be exceeded within these water bodies of the state. The result of exceeding these numeric values will result with the state of Florida being fined by the EPA.
Common pollutants typically found in storm water flow can include sediment, foliage, litter, nutrients, metals, hydrocarbons, and bacteria. Because pollutants vary significantly in their respective physical characteristics, a variety of techniques are applied to better address difficulties with capturing the pollutants. For example; capturing hydrocarbons, which are typically in a liquid state, require a different technique than capturing litter which are in a solid state.
Physical filtration whether by means of filtration media or a screen can be an effective technique for capturing solids. Contained within and attached to the solids is a wide spectrum of pollutants of concern that can range from nutrients, to litter, to bacteria. A difficulty with physical filtration is that the collected solids can become so numerous that the rate of filtration is slowed to an ineffective flow rate. Servicing of the filtration media is typically required, and the servicing often requires the media to be replaced as a part of servicing.
While media filtration of stormwater can be effective for pollution removal, long contact time with the media is required. Essentially, the rate of treatment for media is very slow and requires a lot of time to achieve significant removal efficiencies. The flow rates for stormwater are commonly very high and typically exceed the capabilities of media filtration. Using smaller particles of media will enhance the effectiveness of the media, however, smaller particles are problematic because they tend to clog easily and quickly. Clogged media filter will often flow no water and will require servicing before function is restored. The frequency required for servicing fine granular media, and the costs associated with servicing makes using fine granular media impractical.
Applying polymers to stormwater flow is a type of stormwater treatment that can be adapted to high flows. Polymers applied to water purification techniques have been use for over 75 years. Common applications for polymers include drinking water purification, erosion control, sanitary sewer processing, dredging, lake water quality enhancement, and more.
Polymers generally work in the following way: Solids that are sufficiently small to not settle easily have a tendency to maintain suspension in the water column. These solids typically have a negative charge which enables them to repel from each other, and they are kept in motion by the phenomenon known as Brownian motion. Brownian motion maintains the suspension of these particles in the water column, relatively equally diffused throughout the water. These particles often carry a wide spectrum of pollutants of concern.
Polymers act on these tiny particles to neutralize their charge. Once the charge on these particles have been neutralized, random motion of the water will direct the particles to bump into each other and cohesion will coagulate these small particles to form larger particles. As the particles increase in size the diffusion energy that previously enabled Brownian motion becomes insufficient to maintain suspension of these particles in the water column.
As a result, these larger particles will settle out of the water column and prevented from being transported to a receiving body of water. In addition, an unused portion of the polymer application will find its way into the receiving downstream water body where it will act to enhance the water quality there.
Problems exist with using polymers for stormwater applications. If the stormwater is overdosed with polymers the water can become toxic and affect a receiving body of water in a negative way. Overdosing is a concern when applying liquid polymers. Liquid polymers require significant human oversight to avoid overdosing a stormwater flow which adds significant costs to liquid applications of polymers. To overcome the need for human oversight of polymer dosing, polymers have been engineered into solid logs that dissolve in flowing water.
These polymer logs will not dissolve unless water is flowing across the surface of the logs. These surfaces of the polymer logs erode from the water flow, and as the logs continue to erode polymers are released into the water flow. Dosing concentration is a factor of water flow rate and the number of logs engaged. The problem with polymer logs is that they can become covered by solids and blinded so that the water flow does not erode the surfaces of the log. Grass and leaves are especially likely to quickly blind a polymer log, and most stormwater drain pipes commonly convey large quantities of grass and leaves.
Another problem with managing polymer logs is servicing and replacing the polymer logs as they erode away over time. A critical element for any stormwater treatment system is being able to service the system quickly and easily. The longer it takes to service a stormwater treatment system, the more money it will cost for both manpower and service equipment.
In addition, there is an element of servicing that is centered around the safety of the service technicians. For a service technician to enter a stormwater treatment vault, OSHA requires the service technicians to adhere to protocol referred to as the confined space protocol. The protocol requires the service technician that enters the vault to be equipped with a significant amount of specialized equipment. The confined space protocol also requires more personnel to be involved in the process, and a detailed report that a confined space entry took place must be submitted to an administrator. If the service technicians can complete the servicing without having to enter the vault, the additional manpower and time spent can be avoided. It is important to be able to service and/or replace the polymer logs quickly and easily without requiring a service technician to enter the confined space of a vault.
Another problem to overcome when using polymer logs is that they can dry out when exposed to the air between rain events. When the logs dry out they can develop a hard crust that is resistant to eroding when water is flowing. A polymer log that has a hard crust must be replaced. Polymer logs last longer and function better when permanently submerged in water.
Another problem is the need to remove the gross solids that are retained in a stormwater treatment system. Gross solids that are common in stormwater flow are sediments, foliage, and litter. Once the stormwater flow has been polymerized the amount of solids retained in the treatment vault will increase. Being able to quickly and easily remove gross solids from a stormwater treatment system, without having to send a technician into a confined space, will save money and conserve labor.
Thus, the need exists for solutions to the above problems with the prior art.